How Ion-Selective Electrodes Work

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                      How Ion-Selective Electrodes Work
Some Basics

There are two different types of Electrical Conductivity:
1) In Metal s the electric current is carried by Electrons.
2) In Liquids the electric current is carried by Ions

E very Electrochemical Process (Galvanic Cell, Electrolysis, Electro-Analysis) involves both these
types of conductivity. The junctions where they meet and trans fer the electrical charge are referred to
as Metal-Liquid Interfaces. These interfaces were originally called Electrodes, but now this term is
also used for various other devices such as welding electrodes or electro -cardiogram electrodes.
At the Metal-Liquid interface there is an exchange of Electrons in one or other direction (details can be
found in standard chemistry text books, in sections on Galvanic or Electrolytic Cells.
(NB: Galvanic [Voltaic] Cells generate electricity; Electrolytic Cells consume electricity).

For example, in a Copper-Silver Galvanic Cell, on one electrode an Oxidation reaction takes place:
      Cu (metallic)  Cu 2+ (ionic, in solution) +2 e-
on the other electrode a Reduction reaction takes place:
      Ag+ (ionic, from solution) + e-  Ag (metallic - deposited on electrode surface)

This explains how the electric current in the wire (Electrons) becomes a current in the liquid (Ions).


The Electrochemical Circuit for an Ion Selective Electrode measurement.

An ISE (with its own internal reference electrode - more details later) is immersed in an aqueous
solution containing the ions to be measured, toget her with a separate, external reference electrode.
(NB: this external reference c an be completely separate or incorporated in the body of the ISE to form
a Combination Electrode.) The electrochemical circuit is completed by connecting the electrodes to a
sensitive milli -volt met er using special low-noise cables and connectors. A potential difference is
developed across the ISE membrane when the target ions diffuse through from the high conc entration
side to the lower concentration side (a detailed description follows later).




     Figure adapted from that by Wojciech Wroblewski at CSRG, University of Warsaw, Poland.
             (Copied from http://www.ch.pw.edu.pl/~dybko/csrg/tutorials/ise/index.html)
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General principle of ISE analysis

At equilibrium, the membrane potential is mainly dependent on t he concentration of t he target ion
outside the membrane and is described by the Nernst equation (see Glossary at www.nico2000.net).
Briefly, the measured voltage is proportional t o the Logarithm of the concentration, and the sensitivity
of the electrode is expressed as the electrode Slope - in millivolts per decade of concentration. Thus
the electrodes can be calibrat ed by measuring the voltage in solutions containing, for example, 10ppm
and 100ppm of the target ion, and the Slope will be the slope of the (straight) calibration line drawn on
a graph of mV versus Log concent ration.
          i.e. S = [ mV(100ppm) - mV(10ppm) ] / [Log100 - Log10]
Thus the slope simply equals the difference in the voltages - since Log100-Log10 = 1.
Unknown samples can then be determined by measuring the voltage and plotting the result on the
calibration graph.

The exact value of the slope can be used as an indication of the proper functioning of an ISE and t he
following are typical values:

        Monovalent :     Cations +55 ± 5, Anions -55 ± 5
        Divalent :       Cations +26 ± 3, Anions -26 ± 3

The Function of the Reference Electrode

The membrane potential cannot be measured directly. It needs a Metal-Liquid interface (or a met al-
solid solution interfac e in modern "all -solid-state" ISEs) on both sides of the membrane. Theoretically
these could just be metal wires immersed in the solutions. But the electrical potential on many simple
metal-liquid junctions is not stable; thus the need for a so-called reference system on both sides of the
ISE membrane, with a particular metal-liquid interface which is known to have a stable potential. The
magnitude of t his potential need not be known because it is the same for all measurements of
standards and samples and is thus eliminated during the calibration process.

Nevertheless, it must be not ed that this potential, plus any others that may be generated at any or all
of the metal-liquid or liquid-liquid junctions in the circuit, is the value which is seen when the electrodes
are immersed in pure water or any other solution which does not contain the target ion. This explains
why the measured voltage is not expected to be zero when no target ion is present and also why it is
not necessarily always positive when the target ion is present - it all depends on the difference
between the ISE voltage and the sum of all the other voltages in the circ uit. For example, for a
monovalent positive ion, the voltage could be -25 mV in 10ppm and +30mV in 100ppm (or even -60
mV in 10ppm and -5mV in 100ppm) but this still gives a slope of + 55mV per decade of conc entration
and indicates that the ISE is functioning correctly. Reversing the charges above would describe the
situation for a monovalent negative ion.

It should be noted here that immersion in pure wat er should be avoided becaus e it tends to leach out
the target ion from the ISE membrane. This, together with the inherent instability of the liquid junction
potential of the reference electrode, will cause an unstable voltage to be measured in pure water and
require the ISE membrane to be re-equilibrated in a high concentration " pre -conditioning" solution
before it will give stable readings again.

In practice, the most common reference system is a silver wire coat ed with solid silver chloride and
immersed in a c oncent rated solution (known as the " filling s olution" ) of potassium chloride sat urat ed
with silver chloride. The reference electrode is a half-cell that provides a constant potential whic h is
dependent only on the concentration of c hloride ions in the filling solution. The reversible Redox
reaction involves the chloride atoms in the solid silver chloride (plated on t he silver wire) receiving an
electron and the chloride ion going into solution, and vice versa. This electrode will give a constant
potential of +205 mV (relative to t he Standard Hydrogen Electrode) with a saturated KCl/AgCl solution
at 25°C.
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Electrochemical Processes in the Membrane of an ISE
There are various different charge-transfer processes, both outside and inside the membrane for the
various different membrane types, and many of these are highly complex and poorly understood in
detail. For ex ample, even the apparently simple glass membrane of a pH electrode, which has
traditionally been thought of as involving the passage of Hydrogen (H+), or possibly hydroxonium
(H3O+ ) ions, has recently been shown, by radioactive tagging experiments, to involve only the
movement of Sodium (Na+) ions !

The following descriptions of the Calcium and Fluoride ISEs are typical examples of the basic
principles of ion-selective membrane processes. Nevert heless, it must be noted that these processes
may be far more complex than those described and may involve several layers of ions at each phase
junction.


Principle of Operation of the Calcium ISE
A Calcium ISE has a PVC membrane which is impregnated with an organic molecule which selectively
binds and transports Ca2+ ions, and contains an int ernal solution with a fixed concentration of calcium
chloride - added to the K Cl / AgCl solution of the internal reference system. (Note that in modern all -
solid-state ISEs the internal "solution" is in a solid form). Initially when this electrode is immers ed in a
sample solution containing Ca2+ ions, the potential difference across the membrane is zero - bec ause
there are, on both sides of the membrane, solutions where the electrical charges are balanced (i.e.
they contain equal numbers of cations and anions). But very soon after immersion, calcium ions will
start to diffuse across the membrane, from the side with the higher calcium concentration to the side
with the lower calcium c oncent ration. (NB: for the purposes of this explanation it is convenient to
assume that the flow of ions is from the test solution into the ISE.)

As the positive calcium ions are transported ac ross the membrane by the diffusion pressure, there is a
build up of positive charge (cations) on the inside of the membrane and a corres ponding increase in
negative c harge (anions) outside. These charges on the membrane surface mean that an electrical
potential difference is established across the membrane.
This potential difference causes the calcium ion migration to slow down and finally stop when t he
diffusion pressure due to the difference in concentration is exactly balanced by the electric field effects
due to the fact that similarly charged particles repel one anot her. The potential difference at
equilibrium is the membrane potential.

Inside the ISE, the build up of positive charge at the membrane surface causes silver ions in t he
internal reference system to lose their charge (by receiving electrons from the silver wire) and be
deposited on the wire. Thus electrons are drawn through the external wiring from the meter and
thence from the external reference electrode. Here, chloride ions are attracted to the silver chloride-
coated wire and give up their electrons by combining with silver at oms in the wire, and potassium ions
flow out int o the sample solution through the porous frit (labelled liquid junction in the diagram) to
compens ate for the positive charge deficiency caused by the loss of calcium.

At equilibrium, the electron flow ceases, i.e. there is no current, - but there are residual voltage
differenc es at each metal -liquid, solid-liquid, solid-solid and liquid-liquid junction - in addition to the
membrane potential and the reference electrode stable voltage. The measured potential difference,
(in millivolts) is the s um of all these potentials. In theory, during calibration with standard solutions of
known concentration, and during sample measurement, only the membrane potential is changed so
that the other voltages can be ignored. In practice some of these vary a little - particularly at the
porous frit (liquid junction) of t he reference electrode - and are one of the sources of error in ISE
measurements.
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Principle of Operation of the Fluoride ISE.
In the c ase of the F ISE, the ion -selective membrane is a single crystal of Lanthanum Fluoride (LaF 3)
doped with Europium Fluoride (EuF2) which produces holes in the crystal lattice through which F ions
can pass. When immersed in a fluoride solution and connected via a voltmeter to an AgCl/KCl
external reference electrode immersed in the same solution, the negative F ions in the solution pass
through the crystal membrane by normal diffusion from high concentration to low concent ration until
there is an equilibrium between t he force of diffusion and the reverse electrostatic force due to
repulsion between particles of similar charge. On the other side of the membrane there is a
corresponding build-up of positive ions.

The build up of negative F ions on the inside of the membrane is compensat ed for by Cl ions in the
internal referenc e solution becoming neutralis ed by combinin g with the A g/AgCl wire, and electrons
are thus forced through the external wire t o the voltage measuring device (ion meter or c omput er
interface). The other terminal of the voltmeter is connected to the A g/AgCl wire of the external
reference electrode. Here, t he influx of electrons causes Ag ions in the filling solution to accept
electrons and deposit on the silver wire and, consequently, Cl ions to flow out into the sample solution.

Note that, in general, depending on the conc entrations inside and outside the membrane and which
ion is being measured, all the reactions described above could occur in the opposite direction.


Useful References containing recent research:

David C. Harris (2001) Exploring Chemical Analysis, 2nd Ed. ISBN 0716735407

Skoog, West, Haller & Crouch (2000), Analytical Chemistry, 7th Ed. ISBN 0030202930

Garry D. Christian (1994), Analytical Chemistry. ISBN 0471305820


                                              CCR/HK/Nic o2000/Sept. 2004/ Last Update 19 Jan. 2005

				
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